I get a lot of calls about cooling system problems, and I always ask, “What have you been adding?” Truth is, if you don’t have the pink G12 coolant, just add water and have someone test your system for leaks as soon as possible. Your cooling system level should remain fairly constant over time. If your warning light comes on, you either have an overheat condition, or the coolant is low and it needs to be checked immediately.
If your coolant looks rusty or brown like in this photo, it may be contaminated from mixing pink G12 OE coolant and green antifreeze. In this case, the cooling system should be pressure tested, inspected, and flushed to ensure the heater core and radiator work properly.